Laser diodes are widely used in the field of optical disk drive technology, and also in the field of digital transmission over optical fibers. Such laser diodes have the drawback of showing large variations in their luminous output power due to temperature changes in, and aging of, the laser diode.
FIGS. 1A and 1B illustrate the effects of temperature and aging upon the output of a laser diode. FIG. 1A depicts the variations of the characteristic curve of a laser diode as a function of the temperature. For a temperature of T=20.degree. C., the curve C1 is obtained and for a temperature of T=60.degree. C., the curve C2 is obtained. It can be observed that, for a constant current I to the laser diode, the optical power emitted when the temperature is equal to 20.degree. C. is greater than the optical power emitted when the temperature is equal to 60.degree. C.
FIG. 1B depicts plural characteristic curves of laser diodes, one of which corresponds to a new laser diode and another to a laser diode at the end of its lifetime. These characteristic curves have been plotted for a constant temperature equal to .theta..degree. C. It can be observed in FIG. 1B that the optical power emitted, for a given current I to the laser diode, is weaker when the laser is at the end of its lifetime.
Laser drivers are typically controlled with an analog circuit that has at least two external resistors: one to adjust the bias current and one to adjust the maximum power output. FIG. 2 depicts the typical current wave form provided to a laser diode. The current Ibias is set to approximately the threshold current at which the laser lases. The modulation current Imod is set as the difference between the required peak current Ipeak and the bias current, i.e., Imod=Ipeak-Ibias.
During manufacture of a laser driver, the bias current Ibias is set by providing an external resistor and adjusting it until the laser diode barely lases. In the manufacturing environment, this resistor is trimmed by a laser to adjust it to its final value, which results in a bias current approximately equal to the threshold current. Similarly, to set the peak output power, i.e., the peak current to the laser, another external resistor is provided and adjusted until the peak output power of the laser diode is achieved. This resistor is also trimmed with a laser to its final value.
There are several problems associated with the typical laser driver. It requires external resistors and laser trimming, which consumes space otherwise available for other circuitry and complicates the manufacturing process, respectively. More importantly, the typical laser driver fails to compensate for the effects of temperature and aging, discussed above, upon the laser diode. Consequently, laser diode controllers have been developed which compensate for temperature-induced changes and aging-related changes in the laser diode.
One such method involves carrying out analog regulation of the luminous power by the laser diode. This is done by comparing a signal representing the luminous power (as obtained from a monitor photodiode) against a reference value to obtain an error signal. This error signal is used to modify the current that is supplied to the laser diode. One way to implement this is to set the modulation current Imod equal to a constant value and then adjust the bias current Ibias to keep the detected luminous power equal to a constant value. This requires the use of two external resistors to set the values of Imod and desired luminous power.
Another way to implement this method is to set the bias current Ibias equal to a constant value and then adjust the modulation current Imod to keep the measured luminous power equal to a desired luminous power. Again, this requires the use of two external resistors. A particular way to do this is to control the modulation current Imod to be Imod=k1+k2*Ibias. Trim resistors are used to set the coefficient k1 and the desired luminous output power. The coefficient k2 is set in an internal register. Though the latter technique is the best of the implementations discussed above, it still requires the use of trim resistors.
Another laser diode power controller has implemented a more robust analog linear, i.e., proportional-integral (PI), control algorithm using digital logic, but this has required the measurement of both the maximum output power and minimum output power of the laser diode. Such a controller is implemented on a single integrated circuit using BiCMOS technology. The bipolar transistor technology is used for the current-source portions of the circuitry because it is fast and can supply large amounts of current.
As is known, designers of integrated circuits are under tremendous pressure to continually increase the number of discrete functional device, i.e., groupings of transistors, fabricated on a single chip. As such designers of high current-sourcing circuitry have embraced bipolar technology, and with it BiCMOS technology, because the bipolar technology consumes much much less surface area of a chip than would be consumed by the number of NMOS, PMOS or CMOS transistors need to source a comparable amount of current. The CMOS technology has long ago been discarded for use as high current-sourcing circuitry because it cannot comply with the design demands of higher integration, mentioned above. Moreover, designers have not yet made CMOS circuitry perform at the speeds obtainable with bipolar technology, making it even less desirable to designers of high current-sourcing circuitry.
The BiCMOS technology has the problem of being significantly more expensive than pure CMOS technology because of its bipolar technology aspect.